Method for the surface treatment of aluminum electrodes for the electrolytic production of zinc,and electrodes thus treated

ABSTRACT

A METHOD IS DISCLOSED FOR THE SURFACE TREATMENT OF ALUMINUM ELECTRODES FOR THE ELECTROLYTIC PRODUCTION OF ZINC SUBSTANTIALLY CONSISTING IN DIPPING SAID ELECTRODES, BEFORE THEIR USE, INTO A BATH COMPRISING AN AQUEOUS SOLUTION OF AT LEAST ONE ACID SELECTED FROM THE CLASS CONSISTING OF HYDROFLUORIC ACID AND FLUOSILICIC ACID. THE CONCENTRATION OF THE ACID IN THE TREATING BATH IS BETWEEN 1 TO 20 G./1., AND THE DIPPING TIME IS BETWEEN 5 AND 500 SECONDS.

United States Patent 3,689,386 METHOD FOR THE SURFACE TREATMENT 6FALUMINUM ELECTRODES FOR THE ELEC- TROLYTIC PRODUCTION OF ZlNC, AND ELEC-TRODES THUS TREATED Giovanni Scacciati and Giovanni Lanfranco, Turin,Italy, assignors to Centro Ricerche Metallurgiche S.p.A., Turin, ItalyNo Drawing. Filed Nov. 30, 1970, Ser. No. 93,889 Claims priority,application Italy, Dec. 2, 1969, 25,229/ 69 Int. Cl. C22d 1/22'; B0111/00; C23c I/08 US. Cl. 204-114 4 Claims ABSTRACT OF THE DISCLSSURE Amethod is disclosed for the surface treatment of aluminum electrodes forthe electrolytic production of zinc substantially consisting in dippingsaid electrodes, before their use, into a bath comprising an aqueoussolution of at least one acid selected from the class consisting ofhydrofluoric acid and fluosilicic acid. The concentration of the acid inthe treating bath is between 1 to 20 g./l., and the clipping time isbetween and 500 seconds.

The present invention relates to a method for the surface treatment ofthe aluminum electrodes used for the production of zinc by electrolysis,in order to eliminate, or at least reduce to a minimum, the harmfuleffect of fluorine present in the electrolytic solution. The inventionalso relates to the aluminum electrodes thus treated.

It is quite well known that one of the most diflicult problems in themetallurgy of zinc by electrolysis arises from the presence of fluorinein the electrolyte. In fact, when the fluorine content exceeds certainvalues, there occurs the phenomenon of the adhesion of the zinc depositsto the aluminum cathodes. This adhesion may be such as to make it verydiflicult-if not indeed impossibleto bring about the detachment of thezinc deposit; or, as it is commonly called, the stripping of the zincdeposit.

Since the greatest part of the zinc ores contains fluorine in more orless great quantities, this problem is common and always present in allthe electrolytic zinc plants. Often there occur production losses and,at any rate, the control of the fluorine invariably represents a costlyand difficult problem. Moreover, certain zinc ores, excellent from anyother point of view but containing high percentages of fluorine, cannotbe accepted for the metallurgical treatment via electrolysis.

The adhesion phenomenon is ascribed to the pickling action which thefluorine present in the electrolyte exerts on the surface of thealuminum at the starting stage of the electrolysis; that is, when saidsurface is not yet completely covered by the depositing zinc. Thepickling action consists in destroying the oxide film that lies on thealuminum surface and which hinders the adhesion of the zinc deposit.

The phenomenon manifests itself in a different manner in differentplants, depending on the electrolysis conditions, and is probablyrelated to numerous other factOrs.

Therefore, so far it has been impossible to correlate the effect of thefluorine with its concentration in the electrolyte; for example, thereexist plants that cannot tolerate more than 15-20 mg./l. of fluorine,and others that on the contrary operate with 200 mg./l. and more offluorine.

In any case, a sudden increase of fluorine, resulting in fluorinecontents above the customary levels, is the cause of the difficulty orimpossibility of the zinc stripping operation.

Different proposals for eliminating the harmful effect resulting fromthe fluorine have been studied and applied but none so far has yieldedresults satisfactory in all cases. One of the most frequently usedmethods is that of dipping for some time the electrically insulatedaluminum cathodes into the electrolytic cell. This expedient is,however, often inadequate and, at any rate, rather burdensome in as muchas it disturbs the normal operation of the electrolysis and results inhigh labor costs.

Thus, an object of this invention is to provide a method for the surfacetreatment of aluminum cathodes such as to eliminate in all cases, or atleast to reduce to a minimum, the above-cited harmful effects of thefluorine.

Another object of this invention is to provide a cheap method, capableof easy fulfillment, for the treatment of aluminum cathodes.

These, and still other objects that will appear more clearly from thedetailed descriptionthat follows hereunder, are conveniently achieved bya method for the surface treatment of the aluminum electrodes for theelectrolytic production of zinc; said method, according to thisinvention, substantially consisting in dipping said electrodes, beforetheir use, into a bath including an aqueous solution of at least oneacid selected from the class consisting of hydrofluoric acid andfluosilicic acid.

The dipping times may vary within wide limits, depending on the desiredeffect, for instance from 5 to 500 seconds. For reasons of maximumproductivity, preference will be given to dipping times as short aspossible.

Likewise, the concentration of the acid used in the treatment bath mayvary within rather wide limits; thus, for instance, use can be made of abath having an acid concentration between 1 and 20 g./l.

The protective action which the above-indicated treating bath developson the aluminum electrodes, thus avoiding or reducing to a minimum theetching by the fluorine contained in the electrolytic solution, and theconsequently high adherence of the zinc deposited on said electrodes,may be explained in the following way, according to a hypothesisbelieved to be correct but not to be taken as limiting the invention.

The dipping of the aluminum electrode into the bath containing the acidcauses the formation on the surface of the aluminum of a thin layer ofsalts of the aluminum fluoride type or of the aluminum fluosilicatetype, depending on the acid used, said salts being more or lesshydrated. These salts show a greater resistance than the aluminum oxideoriginally existing on the surface of the electrode to the aggressiveaction of the fluorine contained in the electrolyte.

The thin film of salts sets up a kind of cushion between the aluminumand the zinc forming thereon during the electrolysis of the zincsolution, thereby hindering the direct contact between the two metalsand thereby facilitating their subsequent separation.

It should be noted that it is not necessary to repeat the dipping intothe acid bath for each electrolytic cycle; the effect of one treatmentmay be enough for 23 successive cycles.

The corrosion of the aluminum cathode which is caused by the treatingmethod according to this invention is quite irrelevant. In fact, it hasbeen found that for an aluminum electrode having 1 sq. m. of totalimmersed surface and for a treatment every 48 hours, in one year a lossof about 10 g. of aluminum occurs based on a total Weight of 5-6 kg. ofimmersed portion. Thus, the treatment does not significantly affect thenormal average life of the aluminum electrodes.

In industrial practice, after each electrolytic cycle (generally during24 or 48 hours) the set of electrodes is carried to the manually orautomatically operating stripping station, where the zinc deposits arestripped from the aluminum cathodes; thereafter these latter are washedwith water and again arranged in electrolytic cells.

The dipping into the treatment bath according to this invention is madeafter the stripping and before the Water-washing. Since the dippingtreatment only takes a few seconds or at most a few ten seconds, itaffects the electrolytic cycle to a quite negligible extent.

In order still better to illustrate this invention, a few examples arenow given. These examples were obtained from a series of tests performedby feeding 4 industrial electrolytic cells, arranged in cascade, withthe electrolyte for zinc additioned with an established quantity offluorides. Each cell had 6 aluminum cathodes. The average operationalconditions were:

Feed into 1st cell: Zn=80 g./l.; H SO =90 g./l.; Outflow from last cell:Zn==60 g./l.; H SO =120 g./l.; Temperature: 40 C.;

Current density: 500 amp./ sq. m.

EXAMPLE 1 The feeding solution contained 50 mg/l. of fluorine. Threecathodes of each electrolytic cell, after having been washed with water,were treated, according to this invention, by dipping them for 60seconds into a solution of 2 g./l. of fluosilicic acid.

After the treatment the cathodes were washed with a jet of water andsubseuently employed in the electrolySlS.

The stripping of the zinc deposited on the thus pretreated cathodes waseasy.

The stripping of zinc from the non-pre-treated cathodes of the samecells was very diflEicult if not impossible.

EXAMPLE 2 The feeding solution contained 50 mg./l. of fluorine. Threecathodes of each cell, after having been washed with water, weretreated, according to this invention, by dipping them for 60 secondsinto a solution of 1.5 g./l. of hydrofluoric acid.

After the treatment the cathodes were washed with a jet of water andsubsequently employed in the electrolysis.

The stripping of the zinc deposited on the thus pretreated cathodes waseasily done.

The stripping from the non-pre-treated cathodes of the same cells wasvery difficult, if not impossible.

What is claimed is:

1. A method for the surface treatment of aluminum electrodes for theelectrolytic production of zinc wherein said electrodes, before theiruse, are dipped into a bath consisting of an aqueous solution of atleast one acid selected from the class consisting of hydrofluoric acidand fluosilicic acid, and thereafter using said electrode as a cathodein a cell during the electrolytic production of mm.

2. A method according to claim 1, wherein the concentration of the acidin the treating bath is between about 1 and 20 g./l.

3. A method according to claim 1, wherein the dipping time in thetreating bath is between about 5 and 500 seconds.

4. An aluminum electrode for the electrolytic production of zinc, thesurface of which is covered by a thin layer of a salt selected from theclass consisting of aluminum fluoride and aluminum fluosilicate.

References Cited OTHER REFERENCES Kirk, R. and Othmer, D. F.Encyclopedia of Chemical Technology, vol. 15, pp. 258-9, 1956. TheInterscience Encyclopedia, Inc., NY.

JOHN H. MACK, Primary Examiner R. I. FAY, Assistant Examiner US. Cl.X.R.

